Plasma chromatograph with internally heated inlet system

ABSTRACT

Response time of drift-cell apparatus for measuring trace gases is improved by heating the drift cell walls and/or the sample inlet to reduce the accumulation of sample substances. Heated filters and electrode structures with tortuous gas paths are also disclosed.

United States Patent Cohen [45] Oct. 10, 1972 [54] PLASMA CHROMATOGRAPHWITH 3,211,996 10/1965 Fox et a1 .....250/4l.9 INTERNALLY HEATED INLET3,502,868 3/ 1970 Gentsch ..250/4 1 .9 SYSTEM 7 3,254,209 5/1966 Fite..250/4l.9 SE

[72] Inventor: gl/lartin J. Cohen, West Palm Beach, FOREIGN PATENTS OR CIO 4 t B 'tai ..250 41.9 73 Assignee: Franklin GNO Corporation, West968912 9/196 n n Palm Beach, Fla. OTHER PUBLICATIONS Filed! 1969 Devicefor Mass Spectrometer Heated .lnlet NOI: System, ROWE, June Related US.Application Data Primary Examiner-James W. Lawrence [63]Continuation-impart of Ser. No. 779,096, Nov. Assistant Emmmerc' Church26, 1968 Attorney-Raphael Semmes 52 us. 01.....250/419 TF, 250/419 0,250/419 5 1 ABSTRACT [51] Int. Cl. ..H0lj 39/34, Bold 59/44 Responsetime of drift cel] apparatus f measuring [58] Fleld of Search "250/419 Strace gases is improved by heating the drift Ce walls 56 R f ed and/orthe sample inlet to reduce the accumulation of 1 e erences It samplesubstances. Heated filters and electrode struc- UNITED STATES PATENTStures with tortuous gas paths are also disclosed.

2,699,505 l/ 1955 Usher et al. ..250/4l .9 8 Claims, 2 Drawing FiguresQIUUOGQOGUIOQU PATENTED'ucI 10 I972 INVENTbR MARTIN J. COHEN ATTORNEYPLASMA CHROMATOGRAPII WITI-I INTERNALLY HEATED INLET SYSTEM REFERENCE TOCOPENDING APPLICATION BACKGROUND OF THE INVENTION This invention relatesto apparatus and methods for performing measurements upon gaseoussamples and more particularly is concerned with improving the responsetime of trace gas measurement apparatus and methods.

The aforesaid copending application discloses trace gas measurementapparatus and methods in which the effects of moisture in the sample aresignificantly reduced by diluting the sample with dry air. While thatapplication is principally concerned with reducing moisture-producedinterference, the system disclosed in the application includes heatingmeans for the measurement chamber and gas inlet.

BRIEF DESCRIPTION OF THE INVENTION The present invention is concernedwith the utilization of heat to improve the response time ofmeasurements performed upon gaseous samples, and it is accordingly aprincipal object of the invention to provide apparatus and methods forsuch purposes.

Briefly stated, typical embodiments of the invention employ a PlasmaChromatograph drift cell, referred to more fully hereinafter, whichreceives a gaseous sample through an inlet. The cell chamber and/or theinlet are heated to reduce wall adsorption of the sample substances. Inone form of the invention, a heated filter is also provided, while inanother form of the invention, heated gas is employed to raise thetemperature of an electrode within the cell.

BRIEF DESCRIPTION OF THE DRAWING The invention will be further describedin conjunction with the accompanying drawing, which illustratespreferred and exemplary embodiments, and wherein:'

FIG. 1 is a diagrammatic longitudinal sectional view of one form of theinvention; and

FIG. 2 is a similar view of another form of the invention.

DETAILED DESCRIPTION OF THE INVENTION Before the invention is describedin detail, reference will be made to the subject of PlasmaChromatography, which is disclosed more fully, for example, in thecopending application of Martin J. Cohen, David I, Carroll, Roger F.Wernlund, and Wallace D. Kilpatrick Ser. No. 777,964, filed Oct. 23,1968, and entitled Apparatus and Methods for Separating, Concentrating,Detecting, and Measuring Trace Gases. The Plasma Chromatography systeminvolves the formation of primary or reactant ions and the reaction ofsuch primary ions with molecules of trace substances to form secondaryor product ions, which may be concentrated, separated, detected, andmeasured by virtue of the difference of velocity or mobility of the ionsin an electric field. The primary ions may be produced by subjecting themolecules of a suitable host gas, such as air, to ionizing radiation,such as beta rays from a tritium source, corona from a multi-point orwire array, electrons produced by photoemission from a cathode, etc. Theprimary ions are subjected to an electric drift field, causing them tomigrate in a predetermined direction through a reaction space into whichthe sample or trace gas is introduced. The resultant collisions betweenprimary ions and trace gas molecules produce secondary ions of the tracegas in much greater numbers than can be produced by mere electronattachment to the trace gas molecules. Secondary ions are also subjectedto the electric drift field and may be sorted in accordance with theirvelocity or mobility.

Referring to FIG. 1 of the drawing, reference numeral 10 designatesgenerally a Plasma Chromatography cell of the type disclosed in theaforesaid copending applications. The cell comprises an envelope 12enclosing a series of electrodes, which may be of parallel planegeometry. Principal electrodes 14 and 16 may be arranged adjacent toopposite ends of the envelope, which may be a circular cylinder, forexample. When the apparatus is used to detect negative ions, as will beassumed for example, electrode 14 will be a cathode and electrode 16 ananode. When the apparatus is used to detect positive ions, thepolarities will be reversed. The cell preferably includes a pair ofshutter grids or ion gates 18 and 20, each of which may comprise twosets of interdigitated parallel wires, alternate wires of each gridbeing connected together to form the two sets. Cathode 14 or the regionof the envelope near this electrode is provided with ionizing means,which may be of the type mentioned previously. Anode 16 may be acollector plate connected to an output device, such as an electrometer,which may be Carry.v Instruments Model 401 (vibrating-reed) type withcurrent sensitivity of IO'" amps. at a time constant of 300milliseconds.

An electric drift field is provided between the principal electrodes 14and !6. This field may be obtained, for example, from a suitable DCpower supply, such as a battery connected across a series-resistorvoltage-divider chain. The positive terminal of the chain may beconnected to ground and the negative terminal to the cathode l4. Anode16 may be connected to ground through the input circuit of theelectrometer. Taps on the voltage divider may be connected to a seriesof guard rings 22 spaced along the length of the envelope 12 to maintainthe uniformity of the drift field.

Adjacent elements of each shutter grid are normally maintained at equaland opposite potentials relative to a grid average potential establishedby the voltage divider. Under these conditions, the grid or gate isclosed to the passage of electrically charged particles. The potentialsources which provide the equal and opposite potentials just referred tomay be part of grid drive circuits for applying grid opening pulses tothe grids 18 and 20. The grids are opened by driving the adjacentelements of each grid to the same potential, the grid average potential,at predetermined instants.

A sample comprising a suitable host gas, such as air, carrying anappropriate gaseous trace substance, such as triethyl phosphite, flowsinto the envelope by means of a gas inlet pipe 24 and flows out of theenvelope by means of gas outlet pipe 26. Any suitable source of flowpressure, such as a fan or pump, may be employed to The primary ionsdrifttoward the anode l6, and in the reaction space between the cathodeand the first shutter grid 18, the primary ions encounter othermolecules (at the rate of l per second in air at atmospheric pressure).A majority of these collisions will be with oxygen, nitrogen, or othernon-reactive molecules. A small fraction of the collisions will be withthe trace molecules of interest. in these cases, the primary ions willinteract with the trace molecules to form secondary ions. Secondary ionswill have, in general, an appreciable difference in mobility from theprimary ions. The pressure in the chamber 12 is maintained at a level(preferably about atmospheric) which ensures that the mean free path ofions in the reaction region is short compared to the'dimensions of thisregion, so that ion-molecule collisions are promoted.

The ion flux at the first shutter grid 18 will consist of primary ionsand possibly several species of secondary ions. A sample of this mixedion population is periodically admitted to the drift region between thefirst and second shutter grids when the first shutter grid 18 is openedby momentarily driving all of the grid wires to the grid averagepotential. The second shutter grid 20 is opened for a predeterminedinterval at a predetermined time after the opening of the first grid.The ions that pass through the second grid drift toward and arecollected by the anode 16. The resultant output current may beintegrated over several cycles to give a measurable current. By scanningthe time of opening of the second grid relative to the first, a drifttime spectrum of the ion population can be obtained inthe output andrecorded (current v. drift time). This permits the various ion speciesto be separated and identified.

Since the details of the Plasma Chromatograph and its operation are notper se the present invention, the foregoing description will suffice asexemplary of the type of instruments to which the present invention maybe applied. The present invention is specifically concemed withenhancing the measurements by ensuring rapid response. It has been foundthat response time is increased by the accumulation of sample substances(e.g., trace gas adsorption) on the walls of the chamber 12 and theinlet 24. It has also been discovered that such accumulation can bereduced to acceptable levels by the application of heat. in theembodiment of FIG. 1, heat is applied to the chamber 12 by means of anelectric heater 28 wound helically about the chamber, although theparticular heating means shown is merely representative. If desired,insulation may be applied about the heater to retain the heat. By thismeans, the chamber may be operated at a temperature of fromapproximately l00 to 200 C., for example, thereby to avoid theaccumulation of relatively low volatility substances within the chamber.

To avoid an accumulation of sample material upon the walls of the inletpipe 24 (which would increase response time), the inlet is also heated.This also permits the entire cell to operate under isothermal conditionsand prevents the cooling of the chamber which would result from theadmission .of cool gas. Heating may be accomplished by heating theincoming gas itself by any suitable means, such as an electric heater 30inserted in the inlet pipe 24. It is desirable to have turbulent fiow,no heater hot spots, and good heat transfer.

An insulated pipe section, such as Teflon or quartz, beyond the heatedportion will preserve the gas temperature. Such a pipe section isindicated diagrammatically at 34. A fine bore inlet tubing to meet theReynolds condition for turbulent flow and/or heated obstructions in thetube (such as the heater itself) yield good heat transfer. Forparticulate filtering purposes, an inlet .pipe including a filter 36 isnecessary. The filter must be heated (as by heat transfer from theheated section 32 or from the heated gas) to avoid sample hold-up.Renewal of the filter can be done by rotating a fresh filter intoposition in the inlet pipe periodically. Typical filters may be made ofTeflon, such as Millipore Corporation Type LS (good to 250 C.) orSoloinert Brand Type UG (good to 200 C.).

if a cold (e.g., room temperature) Plasma Chromatography chamber isemployed, then the principal electrode adjacent to the ionizer (forexample, the cathode electrode) should be heated. This concept isillustrated in FIG. 2, wherein the same reference numerals applied inFIG. 1 are used to designate corresponding parts. The temperature ofcathode 14 of the Plasma Chromatograph is raised by heat transfer fromthe heated gas applied through the inlet 24'. Any suitable means- (suchas the electric heater 30 of FIG. 1) maybe employed to heat the gas. Thecathode structure provides a tortuous or re-entrant path for the flow ofheated gas to facilitate the heat transfer. The cathode structure mayinclude an insulating wall adjacent to the inlet, and'the re-entrantpath may be defined by thin conductive sheets of copper or silver, forexample, the sheet closest to the first grid being a radioactive source.An advantage to using the sample gas to heat the cathode is that highvoltage insulation, which makes for poor thermal conduction and would berequired for direct electric heating of a high potential cathode from agrounded power circuit, is avoided.

While preferred embodiments of the invention have been shown anddescribed, it will be .apparent to those skilled in the art that changescan be made without departing from the principles and spirit of theinvention. For example, special materials having low reactivity, such asTeflon, may be employed in the chamber construction, as well as theinlet tube.

The invention claimed is:

l. A method of detecting a trace substance in a gaseous sample, whichcomprises introducing said sample into a chamber through an inlet pipe,heating said sampie from within said inlet pipe, providing reactant ionsin said chamber, reacting said ions with molecules of said sample insaid chamber to produce product ions of different mobility, applying adrift field to the ions in said chamber, maintaining the pressure insaid chamber at a level such that the length of the mean free path ofsaid ions is substantially less than the dimensions of said chamber,segregating said ions in said chamber in accordance with their mobility,and detecting at least a portion of the segregated ions.

2. A method in accordance with claim 1, wherein said chamber is heatedduring the foregoing steps.

3. A method in accordance with claim 2, wherein the chamber is heated toa temperature of from approximately 100 C. to approximately 200 C.

4. A method in accordance with claim 1, wherein the sample is caused toflow through said pipe under turbulent flow conditions.

5. A method in accordance with claim 1, wherein the heated sample ispassed through a filter for removing particulates from said samplebefore the same is introduced into said chamber and heat is transferredfrom said sample to said filter for elevating the temperature of thelatter.

6. Apparatus for detecting a trace substance in a gaseous sample by ionanalysis, comprising a chamber having a pair of electrodes spaced aparttherein, means,

for introducing said sample into said chamber, means adjacent to one ofsaid electrodes for providing reactant ions to form product ions ofdifferent mobility by reaction of said reactant ions with molecules ofsaid sample, means for applying a drift field between said electrodesfor causing said ions to drift toward the other electrode, means formaintaining the pressure in said chamber at a level such that the lengthof the mean free path of said ions is substantially less than thedimensions of said chamber, means for segregating said ions in saidchamber in accordance with their mobility in the drift field, and meansfor providing an electrical output from said other electrode in responseto at least a portion of the segregated ions, said means for introducingsaid sample into said chamber comprising an inlet pipe having meansinside the pipe for heating the sample from within the pipe.

7. Apparatus for detecting a trace substance in a gaseous sample by ionanalysis, comprising a chamber having a pair of electrodes spaced aparttherein, means for introducing said sample into said chamber, meansadjacent to one of said electrodes for providing reactant ions to formproduct ions of different. mobility by reaction of said reactant ionswith molecules of said sample, means for applying a drift field betweensaid electrodes forcausing said ions to drift toward the otherelectrode, means for maintaining the pressure in said chamber at a levelsuch that the length of the mean free path of said ions is substantiallyless than the dimensions of said chamber, means for segregating saidionsin said chamber in accordance with their mobility in the driftfield, and means for providing an electrical output from said otherelectrode in response to at least a portion of the segregated ions, saidmeans for introducing said sample into said chamber comprising an inletpipe with a filter therein and means within said means for introducingsaid sample for heating said filter.

8. Apparatus for detecting a trace substance in a gaseous sample by ionanalysis, comprising a chamber having a pair of electrodes spaced aparttherein, one of said electrodes having a plurality of spaced wallsdefining a tortuous path through the electrode, means for introducingsaid sample into said chamber along said tortuou s path, means adjacentto said one electrode for PI'OVldlIlg reactant ions to form product tonsof different mobility by reaction of said reactant ions with moleculesof said sample, means for applying a drift field between said electrodesfor causing said ions to drift toward the other electrode, means formaintaining the pressure in said chamber at a level such that the lengthof the mean free path of said ions is substantially less than thedimensions of said chamber, means for segregating said ions in saidchamber in accordance with their mobility in the drift field, means forproviding an electrical output from said other electrode in response toat least a portion of the segregated ions, and means for heating thesample before introduction into said chamber, whereby the heat istransferred from said sample to said one electrode.

1. A method of detecting a trace substance in a gaseous sample, whichcomprises introducing said sample into a chamber through an inlet pipe,heating said sample from within said inlet pipe, providing reactant ionsin said chamber, reacting said ions with molecules of said sample insaid chamber to produce product ions of different mobility, applying adrift field to the ions in said chamber, maintaining the pressure insaid chamber at a level such that the length of the mean free path ofsaid ions is substantially less than the dimensions of said chamber,segregating said ions in said chamber in accordance with their mobility,and detecting at least a portion of the segregated ions.
 2. A method inaccordance with claim 1, wherein said chamber is heated during theforegoing steps.
 3. A method in accordance with claim 2, wherein thechamber is heated to a temperature of from approximately 100* C. toapproximately 200* C.
 4. A method in accordance with claim 1, whereinthe sample is caused to flow through said pipe under turbulent flowconditions.
 5. A method in accordance with claim 1, wherein the heatedsample is passed through a filter for removing particulates from saidsample before the same is introduced into said chamber and heat istransferred from said sample to said filter for elevating thetemperature of the latter.
 6. Apparatus for detecting a trace substancein a gaseous sample by ion analysis, comprising a chamber having a pairof electrodes spaced apart therein, means for introducing said sampleinto said chamber, means adjacent to one of said electrodes forproviding reactant ions to form product ions of different mobility byreaction of said reactant ions with molecules of said sample, means forapplying a drift field between said electrodes for causing said ions todrift toward the other electrode, means for maintaining the pressure insaid chamber at a level such that the length of the mean free path ofsaid ions is substantially less than the dimensions of said chamber,means for segregating said ions in said chamber in accordance with theirmobility in the drift field, and means for providing an electricaloutput from said other electrode in response to at least a portion ofthe segregated ions, said means for introducing said sample into saidchamber comprising an inlet pipe having means inside the pipe forheating the sample from within the pipe.
 7. Apparatus for detecting atrace substance in a gaseous sample by ion analysis, comprising achamber having a pair of electrodes spaced apart therein, means forintroducing said sample into said chamber, means adjacent to one of saidelectrodes for providing reactant ions to form product ions of differentmobility by reaction of said reactant ions with molecules of saidsample, means for applying a drift field between said electrodes forcausing said ions to drift toward the other electrode, means formaintaining the pressure in said chamber at a level such that the lengthof the mean free path of said ions is substantially less than thedimensions of said chamber, means for segregating said ions in saidchamber in accordance with their mobility in the drift field, and meansfor providing an electrical output from said other electrode in responseto at least a portion of the segregated ions, said means for introducingsaid sample into said chamber comprising an inlet pipe with a filtertherein and means within said means for introducing said sample forheating said filter.
 8. Apparatus for detecting a trace substance in agaseous sample by ion analysis, comprising a chamber having a pair ofelectrodes spaced apart therein, one of said electrodes having aplurality of spaced walls defining a tortuous path through theelectrode, means for introducing said sample into said chamber alongsaid tortuous path, means adjacent to said one electrode for providingreactant ions to form product ions of different mobility by reaction ofsaid reactant ions with molecules of said sample, means for applying adrift field between said electrodes for causing said ions to drifttoward the other electrode, means for maintaining the pressure in saidchamber at a level such that the length of the mean free path of saidions is substantially less than the dimensions of said chamber, meansfor segregating said ions in said chamber in accordance with theirmobility in the drift field, means for providing an electrical outputfrom said other electrode in response to at least a portion of thesegregated ions, and means for heating the sample before introductioninto said chamber, whereby the heat is transferred from said sample tosaid one electrode.